High power ultra high frequency load device



11g 7, 1951 A. G. KANDOIAN 25621921 HIGH POWER ULTRA HIGH -FREQUENCYLOAD DEVICE Filed March l0, 1945 10 @lglll E# I #ging INVENTOR. 36' Z5 BY @Hin Patented Aug. 7, 1951 HIGH POWER ULTRA HIGH FREQUENCY LOAD DEVICE Armig G. Kandoian, New York, N. Y., assigner to Federal Telephone and Radio Corporation, New York, N. Y., a corporation of Delaware Application March 10, 1945, Serial No. 582,153

1 Claim.

This invention relates to ultra high frequency systems and more especially to power dissipating or load devices for such systems.

A principal object of the invention is to provide an improved tunable load device for high frequency systems to enable the device to dissipate large amounts of power at highy frequencies.

`Another object is to provide a load device employing a resonant cavity which can be used as a terminating or dummy impedance for ultra highfrequency transmission lines.

Heretofore, in ultra high frequency systems such as those used for frequency modulation and television transmissions and the likeit has been quite common to connect to the transmission line, at a predetermined point or points,.power dissipating loads or impedances. These dummy loads usually y have taken the form` ofceramic coated insulators. Such prior devices have been subject to a number of disadvantages, not the least of which is the liability to burn out, and the liability of the ceramic insulation to crack. Accordingly, it is another principal object of this invention to provide a dummy load device for Vultra high-frequencyuse, wherein the abovenoted and other disadvantages of the usual dummy impedance or resistance, are avoided.

A feature of the invention relates to an improved dummy load or power dissipating device for high frequency transmission lines, which device is of rugged construction and can provide simpler and better impedance matching control to the transmission line over a relatively wide frequencyrange, especially at ultra high-frequencies.

Another feature relates to a power dissipating device suitable for ultra high-frequency systems employing highly resonant transmission lines,

wherein a tunable resonant cavity is provided andY is so designed as to enable it to act as a substantially pure load resistance at ultra high-,frequencies. v I

y A further feature relates toa terminating imand interconnection of parts which constitute a simple, ruggedand easily adjustable resonant cavity load device for high frequency transmis- Sionlipes and. the11ke.

Other features and advantages not particularly set forth will be apparent after a consideration of the following detailed description and the appended claim.

In the drawing, which illustrates certain typical embodiments,

Fig. 1 .shows vone form of device according to the inventionwith direct coupling between the transmission` line and the resonant cavity;

Figs. 2 and 3 are alternative forms of Fig. 1 showing respectively inductive coupling and capa-city coupling between the transmission line and the resonant cavity; and

Figs. 4 and 5 represent further embodiments of the invention.

In general, the invention contemplates a tunable ultra-frequency resonant cavity which is connected to a highly resonanttransmission unit effective at ultra high-frequencies. The cavity is so designed and adjusted that radio frequency power from the line is transmitted into the cavity and is dissipated within the cavity in the form of heat. This is accomplished'primarily by tuning the cavity to resonance and coupling the line to a part thereof having an impedance characteristic substantially equal Ato the surge impedance of the transmission line. In accordance with one aspect of the invention, the coupling is designed and adjusted so that the input impedance of the resonant cavity looking from the transmission line toward the cavity, is substantially a pure resistance and equal to the surge impedance of the transmission line. Under this condition, radio frequency power can be fed into the cavity from the lineand this power is dissipated in the cavity losses. However, two limitations are existent in the amount of p ower .that

can be so dissipated namely the heat dissipation limitation of. the` cavity itself and the voltage maximum, before breakdown, within the cavity:

The. deviceac-cording to the invention provides an `eflicient and ruggedarrangement for meeting both these limitations.

Referring to Fig. 1, there is shown a coaxial cavity device comprising an outer casing I of iron, brass or other suitable metal, and an inner coaxially supported thin-walled metal tube 2. Inv

accordance with one feature of the invention,

- member 2 is of a metal or alloy which has a high metals are iron, nickel, Kovan Nichrome,

etc. This has the eifectiof providing a resonant cavity of low Q. It will be understood however, `that the invention is not necessarily limited to such low Q cavities. The member 2 is attached,

3 as by brazing or welding, to a metal cover 3 which closes off the member I.

Members I and 2 therefore form a coaxial resonant cavity of which member 2 is the central conductor and member I is the surrounding or enclosing conductor. In order to adjust for resonance the member 2 is provided with a plunger 4 which is attached to a shank 5 having an adjusted knob 6 on its exterior end. Shank 5 is threaded and engages the corresponding threaded opening in the cover 3. Thus, by turning knob 6, the effective length of member 2 can be adjusted. The high frequency transmission line may be of highly resonant coaxial type, the center conductor 8 being directly connected to member 2, while the outer pipe or sheath 9 is brazed or threaded to the casing I at the opening III. Line 1 is shown schematically connected to a source II of high frequency power of any well-known kind. Since the tunable dissipating cavity formed by members I, 2 and 4 is closed, its normal heat radiating properties are comparatively high and will therefore dissipate a considerable amount of heat. This heat is generated primarily in the member 2 which, in accordance with the invention, is purposely made of thin gauge metal stock. If desired, suitable head radiating i'lns I2 or the like can be attached to the outer Surface of member I, and additionally a fan or blower may be used to cool member I. Furthermore water cooling may be used on member 2 to carry off the heat generated in this member.

The relationship between the maximum voltage developed in the cavity and the amount of power which the cavity is capable of dissipating is given by where W is the total power y VQ is the quality factor of the cavity Z is the surge impedance of the coaxial type cavity.

It is evident that the voltage V for a given amount of power W may be reduced by reducing the Z0 or Q of the circuit. The latter is by far preferable because it also makes the circuit less critical to adjust-and less critical with frequency. The Q may, of course, be reduced by employing high loss materials, such as iron, nickel, Kovan Nichrome, etc. as above noted.

Another very elective way of reducing the danger of voltage breakdown is to use oil dielectric within the cavity. This will increase the value of voltage at which breakdown will occur by a large factor-e. g. to l0 or more. The presence o f the oil will help the problem in two other ways also, first by increasing the losses in the circuit (by reducing the Q) and second by reducing the size of the cavity because of the increased dielectric constant compared to air.

In a cavity such as shown in Fig. 1. the impedance at any point 0 along the center conductor 2r is a pure resistance given approximately by If for example the line I is a 50 ohm line, and the cavity dissipating unit is to be matched thereto, the value 050 corresponding to the angular length 0 to give 50 ohms pure resistance may be solved as follows:

Z0: 50-'-ZQ sin2 050 (3) 50 2 YS111 50-Z0Q Zo and Q may be determined by routine measurement and by the dimension of the center and outer conductor.

By this means, the point of coupling between the transmission line and the center conductor 2 for proper impedance matching can easily be determined. The connectionmay bedirect 01 a loose coupling made depending upon the single impedance of the line and the structures of this resonant cavity device.

From the above relationship one .can determine how much mismatch to expect as the frequency deviates but always keeping the cavity at resonance by a simple tuning arrangement such as that shown in Fig. 1. It can be seen that the frequency may be Varied before the mismatch will exceed 2:1 on the feeding transmission line '1. If better match is required over that or larger frequency range one more control in addition to the tuning control is desirable. A simple diode, crystal or neon indicator may be used to determine the resonant point for any frequency in the well-known manner.

In practice two cavities of the above type have been built, one with a very high Q of 2500 and the second with a Q of only 60. Lower Qs than 60 are also obtainable. In both of the above cases the match on the feeding transmission line was good over the desired range of 500 to 600 megacycles.

Referring to Fig. 2, there is shown a device similar to Fig. 1 with the exception that the coaxial line conductor 8, instead of being'directly coupled to the cavity is inductively coupled by means of the small inductive loop I3.

Fig. 3 shows another embodiment wherein the coaxial line conductor 8 is coupled to the cavity electrostatically by means of the small condenser plate I4 which is in spaced relation to the member 2.

Referring to Fig. 4, there is shown another embodiment wherein the coaxial cavity comprises a metal container I5 and a thin-walled coaxially mounted tube I6, which is adjustably threaded through the cover I'I, so as to vary the length of the member I6 within the member I5. The container I5 is provided with a filling of oil I8, the level of which isbelow the opening to which the coaxial line I9 is connected. The container I5 may be provided with an oil level indicating 20, and is arranged to be cooled by immersion within a water bath 2l which is supplied withrunning Water through the inlet and outlet pipes 22, 23.

The oil within the cavity reduces the danger of voltage breakdown by a factor of at least 5 to 1 0 and also increases the loss characteristic of the cavity by reducing the Q thereof. It will be understood, of course, that oil may be used in the embodiments of Figs. 1, 2 and 3, in which event the size of the cavity may be reduced for the same amount of loss introduction, because of the increased dielectric constant of oil as compared with air. A

Referring to Fig. 5, there is shown a further embodiment wherein the central thin-walled metal conductor 24 is a reentrant or U-shaped tube so as to provide a through path for the flow of cooling water therethrough. In this embodi ment, the tuning of the cavity is effected by adjusting the electrostatic capacity between the central cavity conductor 24 and the bottom 25 in any suitable way. For example, a variable condenserl 26 may be connected between the'conl ductor 24 and the bottom of the cavity 25. Some mechanical means as indicated diagrammatically at 21 may be used to perform this adjustment. In Fig. 5, the highly resonant coaxial transmission line 28 has its central conductor 29 connected to cavity member 24, and the outer pipe or sheath 30 is connected to the outer member or casing 3| of the cavity.

Various changes and modifications may be made in the disclosed embodiments Without departing from the spirit and scope of the invention as defined in the objects thereof and in the accompanying claim.

What is claimed is:

A power dissipating device for high frequency transmission systems comprising means defining a high frequency resonant cavity provided with one or more walls of metal having pronounced loss characteristics at high frequencies, means for tuning said cavity to resonance, a transmission line having a given surge impedance, and means including an inductive loop coupling said transmission line to a point in said cavity having an impedance characteristic corresponding substantially to the surge impedance of said line.

ARMIG G. KANDOIAN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number 

